Wave-Enhanced Tidal Sediment Transport on the Huanghe (Yellow River) Delta Front

As part of the NSF Coastal SEES program, a team of scientists are investigating processes that link river and coastal sediment transport responsible for morphodynamic change of the Huanghe delta- an excellent study site because of its high sediment load and long history of natural and engineered avulsions. The Huanghe builds rapidly prograding delta lobes into the Bohai Sea that are modified by waves and tides. Most of the sediment is delivered during a relatively short period of controlled high discharge during the summer, forming steep foreset beds seaward of the river mouth. An instrumented tripod was deployed at ~ 13 m water depth, near the base of the foreset beds of the subaqueous delta, during field campaigns in summer 2016 (high discharge, low energy) and winter 2018 (low discharge, high energy). The time-series datasets are of equal duration (11 days), captured the fortnightly tidal transition and provide an excellent opportunity to investigate the separate influence of tides and waves on sediment transport and dispersal. Tides are strongly-mixed semi-diurnal with tidal ranges of ~ 1.2 and 0.5 m during spring and neap conditions (respectively) and currents of ~ 0.5 and 0.25 m/s during spring and neap (respectively) at 1 mab. Tidal currents are adequate to drive resuspension and transport in the absence of waves primarily during spring tides. Fluxes are strongly isobath-parallel, with cumulative fluxes to the northwest. The major differences between the two data sets are that winter cumulative alongshore flux was four times that of the summer, and though the cross-shore fluxes were comparable in magnitude, they were onshore in summer and offshore in winter. Comparing winter time series to the summer: wave events were more frequent (five versus two); stronger (Hs as high as 4.2 m versus 1 m), longer total time above the resuspension threshold (90% of the time series versus 40%) and greatly enhanced bottom concentrations during events (2 g/L versus 200 mg/L). Although prior studies have suggested the importance of winter wave energy for sediment transport on the Huanghe delta, and modeled waves in the Bohai Sea, these are the first field observations and demonstrate the magnitude of the role of waves for enhancing tidal transport and creating favorable conditions for gravity flows.